A three-dimensional quadrupole ion trap includes three electrodes which define a chamber. Two of the three electrodes are virtually identical and, while having hyperboloidal geometry, resemble small inverted saucers. The electrodes which resemble inverted saucers are called end-cap electrodes and are typically distinguishable by a number of holes in the center of each electrode. For example, one end-cap electrode may have a single small central aperture through which ions can be gated periodically, and the other end-cap electrode may have several small centrally arranged apertures through which ions can be ejected from the chamber of the ion trap so as to interact with a detector. (Note that ion traps which utilize external ion sources typically have a single perforation in each end-cap electrode.) The third electrode also has hyperboloidal geometry and is called the ring electrode. The ring electrode is positioned symmetrically between the two end-cap electrodes, and all three cooperate to define the aforementioned ion trap chamber.
The geometries of the electrodes are defined so as to produce a quadrupole field which, in turn, will produce an ion trapping potential for the confinement of ions in an area within the chamber of the ion trap defined by the ion trapping potential. For example, an ion trapping potential can be created from a field generated when an oscillating potential is applied to the ring electrode and the two end-cap electrodes are grounded.
Because a quadrupole ion trap can generate an ion trapping potential for the confinement of ions, it can function as an ion storage device in which gaseous ions can be confined for a period of time in the presence of a buffer gas, such as 1 mTorr of helium gas. For example, as a storage device, the ion trap can act as an “electric field test-tube” for the confinement of gaseous ions, either positively or negatively charged, or both, in the absence of solvent.
One use of the confinement of gaseous ions in such a “test-tube” permits the study of gas-phase ion chemistry. In addition, the ion trap can also function as a mass spectrometer in that the mass-to-charge ratios of the confined ions can be measured. For example, as each ion species is ejected from the chamber of the ion trap in a mass selected fashion, the ejected ions impinge upon an external detector thereby creating a series of ion signals dispersed in time which constitutes a mass spectrum. Ejection of ions from the chamber of the ion trap can be accomplished by ramping, in a linear fashion, the amplitude of a radio frequency (r.f.) potential applied to the ring electrode; each ion species is ejected from the chamber (and thus the area defined by the ion trapping potential) at a specific r.f. amplitude and, because the initial amplitude and ramping rate are known, the mass-to-charge can be determined for each ion species upon ejection. This method for measuring mass-to-charge ratios of confined ions is known as the “mass-selective axial instability mode”.
One area of interest in which the above described ion traps are utilized is the study of large polyatomic molecules such as peptides, proteins, oligonucleotides, carbohydrates, and synthetic polymers. These polyatomic molecules can be studied in ion traps due to ionization methods introduced during the past fifteen years which can produce multiply-charged ions from such large molecules. These methods include electrospray ionization (ESI), massive cluster impact ionization, and matrix-assisted laser desorption ionization (MALDI)). ESI and MALDI in particular have become the ionization methods of choice for most large polyatomic molecules such as those mentioned above. In the case of MALDI, singly charged ions usually dominate the population of ions produced. However, in the case of ESI, multiply charged polyatomic molecules usually dominate the population of ions produced. In addition, the population of multiply charged ions produced with ESI has a distribution, or range, of charge states, all of which are substantially greater than +1 or −1. As such, the population of multiply charged ions produced with ESI has a distribution, or range, of mass-to-charge ratios.
Having a population of polyatomic molecules present in the chamber of the ion trap which represents a range of mass-to-charge ratios can be a drawback. In particular, the charge state of the polyatomic molecule of interest may be spread out over 10–15 different ionic states which results in a plurality of relatively weak signals when the population of multiply charged polyatomic ions is analyzed. For example, each charge state gives rise to one relatively weak mass spectrum signal when the population of polyatomic ions is subjected to the previously mentioned “mass-selective axial instability mode” of mass spectrometry. Accordingly, there is a need for a method of operating an ion trap which addresses the aforementioned drawback.